The present invention pertains generally to the field of spectroscopy. More particularly, the new and useful invention claimed in this document pertains to an apparatus and method for producing a substantially straight image produced by a spectrograph, monochromator, or a similar optical measurement device (collectively in this document, xe2x80x9cinstrumentxe2x80x9d). The apparatus and method for producing a substantially straight instrument image in a spectrally dispersed focal plane is particularly, but not exclusively, useful for inducing image curvature adjustments to spectral data associated with a light or radiation signal (collectively, xe2x80x9clight beamxe2x80x9d) by providing entrance slit configurations to overcome curvature, coma, astigmatism and other optical spectroscopy measurement and instrumentation aberrations to the signal shape (collectively, xe2x80x9caberrationsxe2x80x9d).
Spectroscopy is a general term for the process of measuring energy or intensity as a function of wavelength in a beam of light or radiation. Many conventional instruments, including spectroscopes, include basic features and components such as an entrance slit, a collimator for producing a parallel beam of radiation; one or more prisms, mirrors, gratings, reflectors and similar components for receiving and dispersing radiation through differing angles of deviation based on wavelength; an exit slit or an apparatus for collecting spectral data; and perhaps apparatus for displaying and adjusting the image of the dispersed radiation. At least one application of spectroscopy is to use absorption, emission, or scattering of electromagnetic radiation by atoms, molecules or ions to qualitatively and quantitatively study physical properties of matter.
For purposes of both qualitative and quantitative analyses of physical matter, a light beam may be admitted at varying angles into an instrument through an entrance slit, directed through, past, or at one or more prisms, mirrors, reflectors, gratings and other electronic and optical devices (collectively, xe2x80x9cinstrument componentsxe2x80x9d) producing one or more beams of light or radiation that may be directed at a sample of the physical matter (collectively, xe2x80x9cincident radiationxe2x80x9d) to in turn produce one or more beams from which to measure property characteristics of a sample (collectively, xe2x80x9cresultant beamxe2x80x9d). A resultant beam may provide one or more frequencies associated with the sample, as well as the intensities of those frequencies. The frequencies and intensities may be used to identify chemical characteristics of a sample, and resultant spectral data my be collected by any of a variety of detectors, such as a charge coupled device. Spectral data also may be adjusted by algorithms, mathematical formulae, or other means to present the shape of the resultant beam spectral data on an electronic display such as a computer screen.
Efforts to display and present images of a resultant beam have not always resulted in uniform, predictable results, or in acceptable levels of precision and accuracy of spectral measurements. At least one problem is that spectroscopic measurements may be affected by the instrument itself. One or more of the instrument components may contribute to undesirable instrumentation variabilities that affect spectral data measured by the instrument. Problems associated with instruments and methods used to employ phenomena associated with the Raman shift have been addressed and resolved in exemplary fashion by the apparatus and methods shown in U.S. Pat. No. 6,141,095 issued on Oct. 31, 2000 to Allen, et al., and in U.S. Pat. No. 6,281,971 B1, issued on Aug. 28, 2001 to Allen, et al. Until the present invention, however, at least one problem persisted, namely presenting one or more useful and desirable shapes of images from light beams passing through an instrument. Providing substantially straight images from light beams admitted to an instrument through a curved entrance slit in a spectrally dispersed focal plane in an on-axis instrument, without affecting spectral resolution, has presented a number of perplexing problems.
Converting spectral data that has passed through a number of chemical, electronic, electrical and optical devices, and doing so rapidly and accurately, while providing consistently reliable human-readable images with high resolution, may be affected by the instrument itself, by the shape of the entrance slit of an instrument, by angles of incidence of a light beam entering an instrument, by the wavelength of the light beam, by diffraction orders, by diffraction angles, by the focal length of mirrors and lenses, and by related parameters (collectively, in this document, xe2x80x9cinstrument spectral parametersxe2x80x9d).
Depending on the optical elements selected to configure an instrument, a dispersive instrument may be either on-axis or off-axis. An on-axis dispersive spectrometer, however, generally is one based primarily on lenses, although such an instrument also may include one or more mirrors. As used in this document, the term xe2x80x9con-axisxe2x80x9d generally refers to an instrument in which the optical elements are oriented so that the axes of the optical elements are substantially co-linear with the center of a light beam passing through or past the optical elements. Use of a dispersive on-axis instrument having a straight or substantially straight entrance slit tends to yield a curved image. In off-axis spectrometers, image curvature may be caused at least by the arrangement and orientation of the optical elements which are oriented so that the axes of the optical elements are not substantially co-linear with the center of a light beam passing through or past the optical elements. The term xe2x80x9coff-axisxe2x80x9d as used in this instrument generally refers to an instrument that is primarily mirror based, and in which the optical elements are oriented so that the axes of the optical elements are not substantially co-linear with a light beam passing through or past the optical elements. In an off-axis instrument system, the axis of symmetry with respect to reflective surfaces generally forms an angle with regard to each other and with regard to light reflected from the mirrors. As used in this document, the terms xe2x80x9con-axisxe2x80x9d and xe2x80x9coff-axisxe2x80x9d are used not to limit the scope of the present invention, but simply to clarify industry differences in the way instrument components, particularly the orientations and arrangement of optical elements, may be assembled. FIGS. 1A and 1B are useful in appreciating the difference between an off-axis mirror based instrument, and on-axis lens-based instrument. As shown in FIG. 1A, an off-axis mirror-based instrument is shown with a light beam entering through an entrance slit. The light beam is collimated by an off-axis Mirror A, diffracted by a Grating, then focused on a detector by a second off-axis Mirror B. A conventional on-axis lens-based instrument is shown in FIG. 1B, which shows a light beam entering an entrance slit of the instrument. The light beam is collimated by an on-axis Lens A, dispersed by a Dispersive Element, in this case a prism, and focused on a detector by a second on-axis Lens B.
Mirrors are reflective and therefore difficult to use in an on-axis environment. Accordingly, mirrors are usually used in an off-axis environment, but are known to introduce one or more image aberrations. Lenses, on the other hand, are transmissive, and therefore may be used in an on-axis environment to substantially eliminate off-axis aberrations. Lenses, however, are not typically used in spectrographs or monochromators where the intent is to cover a wide range of wavelengths because lenses have chromatic aberrations which cause light beams of different wavelengths to have different focal lengths. On the other hand, if an intended application involves a substantially narrow spectral window, so that chromatic aberration is not significant, as in Raman spectroscopy, lenses may be used in an on-axis environment to yield superb imaging quality.
An exit slit in a monochromator, as opposed to an entrance slit, maybe located at a spectrally dispersed focal plane. A single detector may be used to collect one data point at a time of the spectrum. An entire spectrum may be collected sequentially by scanning an instrument component such as a grating or slit. If both the entrance slit and the exit slit are substantially rectangular, a curvature will result in reduced resolution of the resulting image if a significant portion of the exit slit is used during analysis of a sample.
A spectrograph, however, generally does not employ an exit slit. Rather, a multi-channel detector including by way of example, but not of limitation, a charge coupled device, or a linear diode array, may be used to collect a portion of a light spectrum simultaneously. Such multi-channel detectors often include a two dimensional rectangular matrix. Such a detector usually is oriented so that the x-axis dimension coincides with the intersecting line of the dispersion plane and the focal plane, and the y-axis dimension remains parallel to the entrance slit of the spectrograph. Light or radiation falling on the same columns of the detector array may be added together to enhance sensitivity, or may be dissected into sections that represent spectral data emanating from different locations on or along the entrance slit. Because of the induced slit curvature, integrating the beam or resultant beam vertically results in loss of resolution, while dissecting the signal into sections will result in spectral data with shifted wavelength axes.
Various apparatus and methods attempting to overcome these problems, and seeking to produce high spectral resolution of images that result in reliable, accurate, human-readable images that eliminate aberrations, have been suggested. Straight, curved, or even circular entrance slits have been proposed. Within the array of optical spectroscopy measurement instruments, efforts to achieve a straight image from an instrument application have included reshaping either the entrance slit, the exit slit, or both; reforming the shape of instrument components within the instrument, such as curving the image plane in a prescribed manner; orienting the dispersing plane at or in a prescribed attitude or relationship to the image plane; imaging spectral data along prescribed shapes on one or more variously configured imaging planes; segmenting various surfaces of instrument components to later reassemble the image spectral data as an image; varying the wavelength of light or radiation; varying the focal lengths of mirrors in the case of off-axis instruments; employing a multiplicity of slits, one or more of which may be variously configured; rearranging various angles of incidence among instrument components so as to alter the light beam itself; and using one or more iterations of all of those suggested solutions.
Thus, many devices and methods proposed to solve the problem have employed either a straight entrance slit in an on-axis environment, or a curved entrance slit in an off-axis environment. No solution, however, has been suggested that provides an apparatus and method employing a curved entrance slit in an on-axis environment, plus a combination of formulae to adjust the beam to produce a substantially straight instrument image. None of the suggested approaches provide a method and apparatus that induces image curvature adjustments to spectral data associated with a light beam by providing an entrance slit configuration to overcome the optical element problems caused by instrument components and aberrations.
Therefore, a previously unaddressed need exists in the industry for a new and useful apparatus and method for producing a substantially straight instrument image to assist in qualitative and quantitative analyses of a sample. Particularly, there is a significant need for a method and apparatus that provides image curvature adjustments to spectral data associated with a light beam by providing the entrance slit configuration to overcome curvature, coma, astigmatism and other optical spectroscopy measurement aberrations to the signal shape.
Given the conventional solutions for attempting to solve the problems associated with providing a substantially straight image from a dispersive instrument, it would be desirable, and of considerable advantage, to provide an apparatus, and method for making an apparatus, capable of providing substantially straight images in a dispersed focal plane, and which allows use of a variety of detectors, including, without limitation, a two dimensional detector array, all without affecting spectral resolution.
The present invention provides numerous advantages over prior suggested solutions. At least one of the advantages of the present invention is that it provides an apparatus and a method for producing a substantially straight instrument image in a spectrally dispersed focal plane.
Another advantage of the present invention is that it provides the substantially straight image in an imaging spectrometer using on-axis optics and a curved entrance slit to yield the substantially straight slit images.
Yet another advantage of the present invention is that it provides analytical formulae to transform instrument spectral parameters to provide the optimum entrance slit shape for any combination of optical parameters. This advantage is one of the significant advances over the present technology. By applying the present invention, substantially straight images are now instrument independent. Regardless of the optical elements included in an instrument, a designer of the instrument will know the instrument spectral parameters associated with the optical elements of a particular instrument. Application of the formulae included in the present invention to transform instrument spectral parameters will yield a substantially straight image, regardless of the combination of optical elements included in the particular instrument.
The present invention also permits use of a variety of detectors, including a two dimensional detector array, without loss of spectral resolution.
Yet another advantage of the present invention is an apparatus and method for producing a substantially straight instrument image which respectively are easy to use and to practice, and which are cost effective for their intended purposes.
These and other advantages are achieved in the present invention by providing an on-axis instrument that includes a curved entrance slit. In a preferred embodiment of the present invention, a light beam controller is provided that includes a curved entrance slit to admit one or more beams of light. Means operably connectable to the light beam controller are provided for directing a beam of light into the instrument through the light beam controller. The provision of a light beam controller, however, is not a limitation of the present invention. Any number of structural elements may be used in connection with a particular instrument to provide at least one curved entrance slit in connection with the instrument. As used in this document, the term xe2x80x9ccurvedxe2x80x9d is used in contradistinction to the term xe2x80x9csubstantially straight.xe2x80x9d A plurality of instrument components may be mountable in the path of the light beam, as well as means for collecting image data, such as a detector. Formulae and equations for transforming the one or more beams of light to a substantially straight image also are provided.
As stated, an apparatus and method for producing a substantially straight instrument image, according to at least one embodiment of the present invention, may include a light beam controller. The light beam controller is formed with a housing having a distal end and an proximal end. A curved entrance slit is formed in the proximal end of the housing. Any number of alternative curved entrance slits may be employed. The curved entrance slit, for example, may be shaped as an arcuate channel having opposing curve ends, and jaws may be positioned in the arcuate channel. Alternatively, a waveguide shaped as a curved slit may be inserted in the instrument. If the curved slit is formed in a light beam controller, one or more leads may be inserted into the distal end of the light beam controller. The one or more leads may be one or more fiber optic leads operably connectable to the distal end of the housing.
A light beam may be directed into the instrument through the light beam controller. One or more optical devices are contained in the instrument and located in the path of the light beam. The optical devices may include at least one lens. A plurality of lenses may be arranged on-axis. Other optical devices may be included, such as a grating. In addition, one or more detectors are positioned in the path of the light beam for collecting image spectral data from the light beam. By way of example, but not of limitation, a detector may be a multi-channel device for collecting spectral data. The detector also may be a single channel device. In addition, a charge coupled device may be used, as well as a diode.
By applying one or more formulae to the instrument spectral parameters, the shape and configuration of the entrance slit may be configured to produce a substantially straight image. The theoretical basic formula to determine curvature of an image of the slit is the equation:
mxcex=d cos xcex3(sin xcex1+sin xcex2),xe2x80x83xe2x80x83(1)
where m is the diffraction order, xcex is wavelength, d is a distance between adjacent grooves on a grating, xcex1 is an incident angle, xcex2 is a diffraction angle, and xcex3 is the angle formed between a collimated light beam and the dispersive plane, and is related to the collimating lens focal length identified by f1 and the point source height hxe2x80x2 on the entrance slit by the formula:                               tan          ⁢                      xe2x80x83                    ⁢          γ                =                              h            xe2x80x2                                f            1                                              (        2        )            
It will be evident to those skilled in the art that transforming instrument spectral parameters using equation (1) in an instrument having a straight entrance slit will yield a curved image. If, however, a straight image is desired at the detector focal plane, the instrument entrance slit must be curved.
Accordingly, a derived formula and equation may be used for determining the substantially straight image at the detector to determine the required shape of the slit using the equation
x=cy2,xe2x80x83xe2x80x83(3)
where c is a coefficient and       c    =                  m        ⁢                  xe2x80x83                ⁢        λ                    2        ⁢        d        ⁢                  xe2x80x83                ⁢                  f          1                ⁢        cos        ⁢                  xe2x80x83                ⁢        α              ,
and f1 is the focal length of at least one collimating lens.
As will be evident to those skilled in the art, the application of formula (2) suggests that the optimum shape of an instrument entrance slit that will yield a substantially straight image is a parabolic curve.
In addition, however, an approximating formula can be derived from formula (1) that will provide an instrument entrance slit configuration in the form of an arc or portion of a circle whose radius will be determinable by the formula:
r=1/[2c]+1/8ch2,xe2x80x83xe2x80x83(4)
where h is the height of the slit.
Thus, it is clear from the foregoing that the claimed subject matter as a whole, including the structure of the apparatus, and the cooperation of the elements of the apparatus, as well as the method provided, combine to result in a number of unexpected advantages and utilities of the present invention. Those advantages include providing an apparatus and a method for producing a substantially straight instrument image in a spectrally dispersed focal plane. The present invention also provides a substantially straight image in an imaging spectrometer using on-axis optics and curved entrance slit to yield the substantially straight slit images. In addition, analytical formulae may be used to calculate the dimensions of the optimum entrance slit shape for any instrument. The invention permits use of a variety of detectors, including a two dimensional detector array, without loss of spectral resolution.
The foregoing has outlined broadly the more important features of the invention to better understand the detailed description which follows, and to better understand the contribution of the present invention to the art. Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in application to the details of construction, and to the arrangements of the components, provided in the following description or drawing figures. The invention is capable of other embodiments, and of being practiced and carried out in various ways. Also, the phraseology and terminology employed in this disclosure are for purpose of description, and should not be regarded as limiting.
As those skilled in the art will appreciate, the conception on which this disclosure is based readily may be used as a basis for designing other structures, methods, and systems for carrying out the purposes of the present invention. The claims, therefore, include such equivalent constructions to the extent the equivalent constructions do not depart from the spirit and scope of the present invention. Further, the abstract associated with this disclosure is neither intended to define the invention, which is measured by the claims, nor intended to be limiting as to the scope of the invention in any way.
The advantages and other objects of the present invention, and features of such a an apparatus and method for producing a substantially straight instrument image, will become apparent to those skilled in the art when read in conjunction with the accompanying following description, drawing figures, and appended claims. The novel features of this invention, and the invention itself, both as to structure and operation, are best understood from the accompanying drawing, considered in connection with the accompanying description of the drawing, in which similar reference characters refer to similar parts, and in which: